1. Field of the Invention
This invention relates to a zirconia sol and a method for the production thereof. More particularly, it relates to a zirconia sol stable in the neutral to alkaline regions and a method for the production thereof.
2. Description of the Prior Art
Among the conventional methods for the production of a zirconia sol from the aqueous solution of a zirconium salt, the method which comprises causing the aqueous solution of a zirconium salt to react with a basic substance thereby forming a precipitate and deflocculating the precipitate with an acid and the method which comprises pouring a basic substance into the aqueous solution of a zirconium salt to a pH value not enough to induce a precipitate and mixing them thereby forming a sol have been renowned. Further, the method which produces a zirconia type colloid sol by hydrolyzing an aqueous solution containing a zirconium salt by application of heat under normal pressure or under increased pressure have been known to the art.
The species of zirconia sol which are produced by the conventional methods described above are invariably stable only in the acidic regions of pH. In the neutral to alkaline regions of pH, they suffer from increase of viscosity or undergo gelation and can be no longer used as sol. A slurry which possesses stability to withstand aging or exhibits low viscosity cannot be prepared by mixing the conventional zirconia sol stable only in the acidic region of pH with the powder of calcium oxide, magnesium oxide, or other similar base, a composite powder using such salts as components thereof, or a powder containing such salts as impurities. When the conventional zirconia sol is applied as a coating agent to a metallic material, it is not easily used safely because of the problem of corrosion of the metallic material by the acid present in the sol. In the circumstances, a desire has been expressed for a zirconia sol which is stable in the neutral to alkaline regions of pH.
Where a zirconium component is required in the alkaline region, an alkaline zirconium salt such as, for example, zirconyl ammonium carbonate or zirconyl potassium carbonate is used. Particularly, the aqueous solution of zirconyl ammonium carbonate itself or the aqueous solution obtained by hydrolyzing the aqueous solution of zirconyl ammonium carbonate by the application of heat or the addition of an acidic substance possesses the function of a zirconia sol to some extent, it has found utility as in an inorganic binder. This sol which ought to be called a zirconyl ammonium carbonate sol contains ammonium ion and carbonic acid ion in large amounts. The sol obtained by thermal hydrolysis of the aqueous solution of zirconyl ammonium carbonate undergoes gelation during the application of heat and is produced barely in a low concentration. Thus, the sol has inferior strength for an inorganic binder or a coating agent. It further has the disadvantage that it shrinks in a large ratio after a heat treatment.
As described above, the species of zirconia sol obtained by the conventional methods have the problem of poor stability in the neutral to alkaline regions of pH. Though the zirconyl ammonium carbonate is stable in the alkaline region of pH, the aqueous solution thereof or the sol obtained by hydrolysis of the aqueous solution and rightfully designated as a zirconyl ammonium carbonate sol requires for stabilization thereof large amounts of ammonium ion and carbonic acid ion and, therefore, has the problem of persistence of these ions as impurities in the product aimed at.
For the formation of a zirconia coating, various methods represented by such physicochemical methods as vacuum deposition and spattering and the method which comprises applying a coating of a zirconium-containing liquid composition to a surface have been employed. Among the methods mentioned above, the method which resorts to the application of a coating permits relatively easy commercialization on large surfaces and, therefore, has been the subject of study at various research institutes in recent years.
The conventional zirconium-containing liquid compositions of the organic solvent type have used zirconium alkoxides as a substratal component thereof. Unfortunately, zirconium alkoxides undergo hydrolysis very rapidly. The coating liquid of such a liquid composition, therefore, is unstable, deficient in stability to withstand aging during storage, and susceptible to hydrolysis by moisture and carbon dioxide gas present in the air. Thus, it allows preparation of a uniform coating composition only with difficulty. For the same reason, the coating liquid limits the coating conditions greatly and suffers from poor workability and permits formation of a coating devoid of pinholes only with difficulty. As a solution to the problem, the coating composition incorporating therein a hydrolysis-inhibiting agent such as the zirconium alkoxide of a glycol or the zirconium compound obtained by the combination of a zirconium alkoxide and a .beta.-dicarbonyl compound such as the ester of .beta.-diketone or .beta.-ketone has been used as a coating liquid composition. The coatings which are obtained by these methods, however, are liable to suffer from cracking and uneven wall thickness and allow formation of coatings of uniform quality only with difficulty.
An object of this invention, therefore, is to provide a zirconia sol which is stable over a wide range of pH, particularly stable in the neutral to alkaline regions of pH and transparent in appearance.
Another object of this invention is to provide a zirconia sol which exhibits stability in the neutral to alkaline regions of pH, contains ions as impurities only in small amounts, and enjoys high purity and transparency of appearance.
A further object of this invention is to provide a zirconia sol which exhibits stability in the neutral to alkaline regions, contains ions as impurities only in small amounts, and enjoys high purity, a high zirconia concentration, and transparency of appearance.
Yet another object of this invention is to provide a zirconia sol dispersed in an organic solvent.